<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="js/vue.js"></script>
</head>
<body>
    <div class="app">
        <input type="text" v-model="username">
    </div>
    <script>
        const vm = new Vue({
            el: '.app',
            data: {
                username: '张三',
            },
            medhods:{},
            watch:{
                username(newVal,oldVal){
                    // 监听到了数据的变化就会触发 函数
                    console.log(newVal,oldVal)
                }
            }
        })
    </script>
</body>
</html>